1. Field of the Invention
The present invention relates to high-speed data communications cables containing a plurality of transmission media. More particularly it relates to cables having a cable jacket in which each of the plurality of transmission media is separated from the other transmission media, by a plurality of channels, where adjacent channels are offset from one another to increase the distance between the respective transmission media within the adjacent channels, thereby reducing the level of coupling of cross-talk signal interference between the transmission media within the cable jacket.
2. Related Art
High speed data communications cables in current use include pairs of wire twisted together forming a balanced transmission line. Such pairs of wire are referred to as twisted pairs.
One common type of conventional cable for high-speed data communications includes multiple twisted pairs. In each pair, the wires are twisted together in a helical fashion forming a balanced transmission line. When twisted pairs are placed in close proximity, such as in a cable, electrical energy may be transferred from one pair of the cable to another. Such energy transfer between pairs is undesirable and is referred to as crosstalk. Crosstalk causes interference to the information being transmitted through the twisted pair and can reduce the data transmission rate and can cause an increase in the bit error rate. The Telecommunications Industry Association (TIA) and Electronics Industry Association (EIA) have defined standards for crosstalk in a data communications cable including: TIA/EIA-568-A, published Oct. 24, 1995; TIA/EIA 568-A-1published Sep. 25, 1997; and TIA/EIA 568-A-2, published Aug. 14, 1998. The International Electrotechnical Commission (IEC) has also defined standards for data communications cable crosstalk, including ISO/IEC 11801 that is the international equivalent to TIA/EIA 568-A. One high performance standard for data communications cable is ISO/IEC 11801, Category 5.
In twisted pairs, the length of a complete twist between the twisted pairs is known as the twist lay. The direction of the twist is known as the twist direction. If adjacent twisted pairs have the same twist lay and/or twist direction, they will tend to lie more closely together within a cable than if they have different twist lays and/or twist directions. Thus, compared to twisted pairs having different twist lays and/or twist directions, adjacent twisted pairs having the same twist lay and twist direction have a reduced center-to-center distance, and longer parallel run. Therefore, the level of crosstalk tends to be higher between the twisted pairs having the same twist lay and/or twist direction when compared to other twisted pairs having different twist lays and/or twist directions. Therefore, twisted pairs within a cable are sometimes given unique twist lays and twist directions when compared to other adjacent twisted pairs within the cable. The unique twist lay and twist direction serve to decrease the level of crosstalk between the adjacent twisted pairs within the cable.
Shielded cable, although exhibiting better crosstalk isolation, is more difficult and time consuming to install and terminate and is therefore more expensive per installation. Shielded conductors are generally terminated using special tools, devices and techniques adapted for the job.
One popular cable type is Unshielded Twisted Pair (UTP) cable. Because it does not include shielded conductors, UTP cable is preferred by installers and plant managers, as it is easily installed and terminated. However, UTP cable typically fails to achieve the level crosstalk isolation required by state of the art transmission systems, even when varying pair lays and twist directions are used.
Another crosstalk requirement known as "alien crosstalk" is the amount of signal coupling or interference between adjacent or stacked cables. In particular, when the cable are adjacently disposed or disposed one on top of another, there is typically crosstalk between the twisted pairs in each cable. For example, in adjacently disposed cables having a substantially flat configuration, the twisted pairs disposed at one end of each adjacently disposed cable will be in close proximity and will tend to have alien crosstalk that may not be acceptable for state of the art transmission systems.
What is needed therefore is a high-speed data communications cable having a reduced level of cross-talk interference between adjacent twisted pairs within the cable and having a reduced level of alien crosstalk between the twisted pairs in adjacent or stacked cables.